The invention relates to an optical system, in particular a projection-illumination unit used in microlithography, of the type defined in more detail in the preamble of claim 1.
EP 0 678 768 A2 has described an optical system of the type described in the introduction, in which stepping and scanning processes are employed and only a narrow, slot-like strip is transmitted from a mask to a waver. To illuminate the entire field, a reticle and the waver are displaced laterally (scanning).
However, a drawback of the optical system according to this prior art is that the slot geometry results in a rotationally asymmetric illumination replication especially on lenses which are close to the waver. This means that the temperature distribution which results from the inevitable heating of the lens is likewise rotationally asymmetrical on the lens, and therefore, owing to the linear correlation between refractive index and temperature and owing to thermal expansion, image errors, such as for example astigmatism, arise on the optical axis.
A further problem with an optical system of this nature is the so-called compaction effect, i.e. aging caused by the irradiation, and this effect likewise leads to image errors.
In EP 0 678 768 A2, it is proposed to use a lens as a "final control element", in order to correct the image error which is produced by uneven heating of the lens. For this purpose, forces which act in the radial direction are allowed to act on the lens. However, a drawback of this solution is that only compressive forces are generated, resulting in an asymmetric change in thickness.
EP 0 660 169 A1 has described a projection-illumination unit which is used in microlithography and in which objectives are provided with correction elements. These include a pair of lenses which can be rotated about the optical axis. In this case, the refractive power from the shape of the lens is altered by superimposing a cylindrical meniscus shape on a spherical lens.